Method for selecting pmi for non-adaptive harq operation in a mimo wireless communication system

ABSTRACT

A method for transmitting an uplink signal at a User Equipment (UE) in a wireless communication system includes receiving, from a Base Station (BS), an uplink scheduling grant for multi-antenna transmission; transmitting the uplink signal precoded using precoding information included in the received uplink scheduling grant to the BS; and retransmitting the uplink signal to the BS according to Acknowledgment/Negative Acknowledgment (ACK/NACK) corresponding to the transmitted uplink signal. The retransmitted uplink signal is precoded using precoding information included in a most recent uplink scheduling grant or a predetermined precoding matrix if an uplink scheduling grant for the retransmission is not received from the BS.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.P2008-0116735, filed on Nov. 24, 2008, which is hereby incorporated byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for transmitting an uplinksignal in a multiple antenna scheme, and more particularly, to a methodfor acquiring precoding information efficiently and transmitting anuplink signal using the precoding information in a multiple antennascheme.

2. Discussion of the Related Art

The present invention relates to a method for transmitting MIMO is shortfor Multiple Input Multiple Output. Beyond conventional schemes using asingle Transmit (Tx) antenna and a single Receive (Rx) antenna, MIMOuses a plurality of Tx antennas and a plurality of Rx antennas tothereby increase the transmission and reception efficiency of data. Withthe use of multiple antennas at a transmitter or a receiver, MIMO seeksto increase capacity or improve performance in a wireless communicationsystem. The term “MIMO” is interchangeable with “multiple antenna”.

The MIMO technology does not depend on a single antenna path to receivean entire message. Rather, it completes the message by combining datafragments received through a plurality of antennas. Because MIMO mayincrease data rate within a certain area or extend system coverage at agiven data rate, it is considered as a promising future-generationmobile communication technology that may find its use in a wide rangeincluding mobile terminals, relays, etc. With the growth of datacommunication, MIMO is attracting attention as a future-generationtechnology that may overcome a limit on transmission capacity that isalmost reached due to the increased data communication.

FIG. 1 illustrates the configuration of a typical MIMO communicationsystem. Referring to FIG. 1, a simultaneous increase in Tx antennas of atransmitter to N_(T) and in Rx antennas of a receiver to N_(R) increasesa theoretical transmission capacity in proportion to the number ofantennas, compared to use of a plurality of antennas at only one of thetransmitter and the receiver. Therefore, transmission rate is increasedand frequency efficiency is remarkably increased. Given a maximumtransmission rate R_(o) that may be achieved in case of a singleantenna, the increase of channel capacity may increase the transmissionrate, in theory, to the product of R_(o) and R_(i) in case of multipleantennas. R_(i) is a transmission rate increase rate.

For instance, a MIMO communication system with four Tx antennas and fourRx antennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna system. Since thetheoretical capacity increase of the MIMO system was proved in themiddle 1990's, many techniques have been actively studied to increasedata rate in real implementation. Some of the techniques have alreadybeen reflected in various wireless communication standards for 3^(rd)Generation (3G) mobile communications, future-generation Wireless LocalArea Network (WLAN), etc.

Concerning the research trend of MIMO, active studies are underway inmany respects of MIMO, inclusive of studies of information theoryrelated to calculation of multi-antenna communication capacity indiverse channel environments and multiple access environments, studiesof measuring MIMO radio channels and MIMO modeling, studies oftime-space signal processing techniques to increase transmissionreliability and transmission rate, etc.

There are two types of MIMO schemes: spatial diversity and spatialmultiplexing. Spatial diversity increases transmission reliability usingsymbols that have passed in multiple channel paths, whereas spatialmultiplexing increases transmission rate by transmitting a plurality ofdata symbols simultaneously through a plurality of Tx antennas. Takingadvantages of these two schemes by using them in an appropriatecombination is a recent active study area.

To describe a communication scheme in a MIMO system in detail, thefollowing mathematical model may be used.

It is assumed that there are N_(T) Tx antennas and N_(R) Rx antennas asillustrated in FIG. 1. Regarding a transmission signal, up to N_(T)pieces of information can be transmitted through the N_(T) Tx antennas,as expressed as the following vector.

s=[s₁, s₂, K, s_(N) _(T) ]^(T)   [Equation 1]

A different transmit power may be applied to each piece of transmissioninformation s₁,s₂,K,S_(NT). Let the transmit power levels of thetransmission information be denoted by P₁,P₂,Λ,P_(NT), respectively.Then the power-controlled transmission informations ŝ may be given as[Equation 2].

ŝ=[ŝ₁, ŝ₂, Λ, ŝ_(N) _(T) ]^(T)=[P₁s₁, P₂s₂, Λ, P_(N) _(T) s_(N) _(T)]^(T)   [Equation 2]

ŝ may be expressed as a diagonal matrix P of transmit power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & Ο & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\mu \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \lbrack {{Equation}\mspace{14mu} 3} \rbrack\end{matrix}$

Meanwhile, actual N_(T) transmitted signals x₁, x₂, Λ, x_(N) _(T) may beconfigured by applying a weight matrix W to the power-controlledinformation vector ŝ. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel statuses, etc. These transmitted signals x₁, x₂, Λ,x_(N) _(T) are represented as a vector x, which may be determined as

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\\mu \\x_{i} \\\mu \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \Lambda & w_{1N_{T}} \\w_{21} & w_{22} & \Lambda & w_{2N_{T}} \\\mu & \; & Ο & \; \\w_{i\; 1} & w_{i\; 2} & \Lambda & w_{{iN}_{T}} \\\mu & \; & Ο & \; \\w_{N_{T}1} & w_{N_{T}2} & \Lambda & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\mu \\{\hat{s}}_{j} \\\mu \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

w_(ij) denotes a weight for a j^(th) piece of information ŝ_(j)transmitted through an i^(th) Tx antenna and the weights are expressedas the matrix W. W is referred to as a weight matrix or a precodingmatrix.

The afore-mentioned transmitted signal x may be considered in two cases:spatial diversity and spatial multiplexing.

In spatial multiplexing, different signals are multiplexed prior totransmission. Accordingly, the elements of the information vector s havedifferent values. In contrast, the same signal is transmitted in aplurality of channel paths in spatial diversity. As a result, theelements of the information vector s have the same value.

Spatial multiplexing and spatial diversity may be used in combination.For example, the same signal may be transmitted through three Txantennas in spatial diversity, while different signals may betransmitted through the other Tx antennas in spatial multiplexing.

Given N_(R) Rx antennas, signals received at the Rx antennas, y₁, y₂, Λ,y_(N) _(R) may be represented as the following vector.

y=[y₁, y₂, Λ, y_(N) _(R) ]^(T)   [Equation 5]

When channels are modeled in the MIMO communication system, they may bedistinguished according to the indexes of Tx and Rx antennas and thechannel between a j^(th) Tx antenna and an i^(th) Rx antenna may berepresented as h_(ij). It is to be noted herein that the index of the Rxantenna precedes that of the Tx antenna in h_(ij).

The channels may be represented as vectors and a matrix by groupingthem. The vector representation of channels may be carried out in thefollowing manner.

FIG. 2 illustrates channels from N_(T) Tx antennas to an i^(th) Rxantenna.

Referring to FIG. 2, the channels from the N_(T) Tx antennas to thei^(th) Rx antenna may be expressed as [Equation 6].

h_(i) ^(T)=[h_(i1), h_(i2), Λ, h_(iN) _(T) ]  [Equation 6]

Also, all channels from N_(T) Tx antennas to N_(R) Rx antennas may beexpressed as the following matrix.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\mu \\h_{i}^{T} \\\mu \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \Lambda & h_{1N_{T}} \\h_{21} & h_{22} & \Lambda & h_{2N_{T}} \\\mu & \; & Ο & \; \\h_{i\; 1} & h_{i\; 2} & \Lambda & h_{{iN}_{T}} \\\mu & \; & Ο & \; \\h_{N_{R}1} & h_{N_{R}2} & \Lambda & h_{N_{R}N_{T}}\end{bmatrix}}} & \lbrack {{Equation}\mspace{14mu} 7} \rbrack\end{matrix}$

Actual channels experience the above channel matrix H and then are addedwith Additive White Gaussian Noise (AWGN). The AWGN n₁, n₂, Λ, n_(N)_(R) added to the N_(R) Rx antennas is given as the following vector.

n=[n₁, n₂, Λ, n_(N) _(R) ]^(T)   [Equation 8]

From the above modeled equations, the received signal is given as

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\mu \\y_{i} \\\mu \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \Lambda & h_{1N_{T}} \\h_{21} & h_{22} & \Lambda & h_{2N_{T}} \\\mu & \; & Ο & \; \\h_{i\; 1} & h_{i\; 2} & \Lambda & h_{i\; N_{T}} \\\mu & \; & Ο & \; \\h_{N_{R}1} & h_{N_{R}2} & \Lambda & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\mu \\x_{j} \\\mu \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\mu \\n_{i} \\\mu \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \lbrack {{Equation}\mspace{14mu} 9} \rbrack\end{matrix}$

In a 3GPP LTE system attracting attention as a future-generation mobilecommunication technology, the above-described MIMO operation appliesonly to downlink signal transmission from a Base Station (BS) to a UserEquipment (UE). As efforts are continuous to increase transmission rateand achieve diversity gain also from uplink signal transmission, thereexists a need for specifying this uplink MIMO technology in upcomingstandards.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method for selectinga Precoding Matrix Index (PMI) for a non-adaptive Hybrid AutomaticRepeat reQuest (HARQ) operation in a Multiple Input Multiple Output(MIMO) wireless communication system that substantially obviates one ormore problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide to provide a method forefficiently applying a MIMO scheme to uplink signal transmission.

Another object of the present invention is to provide a method foracquiring precoding information efficiently and transmitting an uplinksignal using the precoding information in a MIMO scheme.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, amethod for transmitting an uplink signal in a UE in a wirelesscommunication system includes receiving from a BS a scheduling grantsignal including precoding matrix information to be used for an uplinksignal transmission from the UE, precoding an uplink signal using theprecoding matrix information included in the received scheduling grantsignal, transmitting the precoded uplink signal initially to the BS, andretransmitting the uplink signal at least one time, if the BS fails toreceive the initially transmitted uplink signal. If a scheduling grantsignal is not received for the retransmission from the BS, the uplinksignal is precoded and retransmitted using precoding matrix informationincluded in a latest scheduling grant signal received for transmissionof an uplink signal having an HARQ process number equal to an HARQprocess number of the uplink signal to be retransmitted.

If the scheduling grant signal is not received for the retransmissionfrom the BS and there is not the latest received precoding matrixinformation for an uplink signal having the same HARQ process number ofthe uplink signal to be retransmitted, the uplink signal may be precodedand retransmitted using precoding matrix information included in alatest scheduling grant signal received for transmission of an uplinksignal having the same rank and/or frequency band as a rank and/orfrequency band of the uplink signal to be retransmitted. When adifferent precoding matrix is used according to a frequency bandallocated to the UE, a precoding matrix for an uplink signal having thesame rank and frequency band as the rank and frequency band of theuplink signal to be retransmitted may be used as a precoding matrix forthe uplink signal to be retransmitted. If a precoding matrix applies toa total system band, only a rank may be considered.

If the scheduling grant signal is not received for the retransmissionfrom the BS and there is not the latest received precoding matrixinformation for an uplink signal having the same rank and/or frequencyband as the rank and/or frequency band of the uplink signal to betransmitted, the uplink signal may be precoded and retransmitted usingthe precoding matrix information included in the scheduling grant signalreceived for the initial transmission.

In another aspect of the present invention, a method for transmitting anuplink signal in a UE in a wireless communication system includesreceiving from a BS a scheduling grant signal including precoding matrixinformation to be used for an uplink signal transmission from the UE,precoding an uplink signal using the precoding matrix informationincluded in the received scheduling grant signal and transmitting theprecoded uplink signal initially to the BS, and retransmitting theuplink signal at least one time, if the BS fails to receive theinitially transmitted uplink signal. If a scheduling grant signal is notreceived for the retransmission from the BS, the uplink signal isprecoded and retransmitted using a precoding matrix of a precodingmatrix group preset between the UE and the BS in a sequential order, theprecoding matrix group including one or more precoding matrices.

The wireless communication system may be a synchronous HARQ system andthe UE may transmit the uplink signal using multiple antennas.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 illustrates the configuration of a typical Multiple InputMultiple Output (MIMO) communication system.

FIG. 2 illustrates channels from N_(T) Transmission (Tx) antennas to ani^(th) Reception (Rx) antenna.

FIGS. 3( a) and 3(b) illustrate methods for transmitting an uplinksignal in a MIMO scheme in a User Equipment (UE) according to exemplaryembodiments of the present invention.

FIG. 4 is a flowchart illustrating a method for acquiring precodingmatrix information for a retransmission in a UE, when the UE fails toreceive a scheduling grant signal from a Node B according to anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that canbe implemented according to the invention.

The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details. In some instances, knownstructures and devices are omitted or are shown in block diagram form,focusing on important features of the structures and devices, so as notto obscure the concept of the present invention. The same referencenumbers will be used throughout this specification to refer to the sameor like parts.

A User Equipment (UE) needs to efficiently acquire precoding informationto transmit an uplink signal in a Multiple Input Multiple output (MIMO)scheme. Sets of available precoding vectors or precoding matrices arepreset in the form of a codebook between a transmitter and a receiver,and a Precoding Matrix Index (PMI) is transmitted as precodinginformation between the transmitter and the receiver. The precodingmatrices of the codebook may be grouped into different subsets accordingto channel ranks. To help the understanding of such precoding matrixinformation, a codeword as a transmission unit, a rank, and a streamwill first be described in brief.

In a typical communication system, the transmitter encodes transmissioninformation using a Forward Error Correction (FEC) code prior totransmission so that the receiver may correct channel errors in thereceived information. The receiver recovers the transmitted informationby demodulating the received signal and then FEC-decoding thedemodulated signal. During the decoding, the receiver corrects thechannel errors in the received signal.

Every error correction code has its maximum limit in channel errorcorrection. If a received signal has errors beyond the limit of an errorcorrection code, the receiver cannot decode the received signal toerror-free information. Accordingly, the receiver needs a criterion bywhich it determines whether the decoded information has errors or not.Aside from the error correction, a special coding process is requiredfor error detection. In general, a Cyclic Redundancy Check (CRC) is usedas an error detection code.

CRC is one of coding methods for error detection, not for errorcorrection. Typically, the transmitter encodes transmission informationwith a CRC and then encodes the CRC-coded information with an FER code.The resulting one coded unit is referred to as “codeword”.

In the mean time, the numbers of rows and columns in a channel matrix Hrepresenting channel statuses are determined according to the numbers ofTransmission (Tx) and Reception (Rx) antennas. The number of rows isidentical to that of Rx antennas, N_(R) and the number of columns isidentical to that of Tx antennas, N_(T). Thus, the channel matrix H isof size N_(R)*N_(T).

In general, the rank of a matrix is defined as the minimum of thenumbers of independent rows or columns. Accordingly, the rank of thematrix is not larger than the number of rows or columns. For example,the rank of the matrix H, rank(H) is limited as follows.

rank(H)≦min(N _(T) , N _(R))   [Equation 10]

If the matrix is eigenvalue-decomposed, its rank may be defined as thenumber of non-zero eigenvalues. Similarly, in case of Singular ValueDecomposition (SVD), the rank may be defined as the number of non-zerosingular values. Therefore, the rank of a channel matrix physicallymeans the maximum number of different pieces of information that can betransmitted on given channels.

A different piece of information transmitted in MIMO is referred to as‘transmission stream’ or shortly ‘stream’. The ‘stream’ may be called‘layer’. It is thus concluded that the number of transmission streams isnot larger than the rank of channels, i.e. the maximum number ofdifferent pieces of transmittable information.

The channel matrix H is determined by

# of streams≦rank(H)≦min(N _(T) , N _(R))   [Equation 11]

# of streams denotes the number of streams. One thing to be noted hereinis that one stream may be transmitted through one or more antennas.

As stated before, it is assumed that available precoding matrices arepreset in the form of a codebook between the transmitter and thereceiver. It is also assumed that available subsets of precodingmatrices are predetermined according to channel ranks.

If a subset of precoding matrices with of a specific rank in thecodebook includes a subset of precoding matrices with a rank lower thanthe specific rank, it is said that the codebook satisfies a nestedproperty. Therefore, precoding matrix information for a specific rankmay be derived from a subset of precoding matrices with a higher orlower rank.

Exemplary embodiments of the present invention are based on theassumption that a UE transmits an uplink signal in MIMO in an HARQsystem.

HARQ is a hybrid technology of channel coding and Automatic RepeatreQuest (ARQ) in combination to improve decoding performance byretransmitting an erroneous data block and combining the initialtransmission data block with the retransmission data block. HARQ schemesmay be categorized according to the regularity of retransmissiontimings: asynchronous HARQ and synchronous HARQ. The asynchronous HARQis characterized by a variable retransmission timing, whereas thesynchronous HARQ by a preset retransmission timing. Meanwhile, the HARQschemes are classified into Chase Combining (CC) and IncrementalRedundancy (IR) depending on the types of Redundancy Versions (RVs) usedfor retransmission. In CC, a retransmission data block is identical to apreviously transmitted data block, thus resulting in a Signal-to-NoiseRatio (SNR) gain. In contrast, IR achieves a coding gain byretransmitting a data block including data of a different RV from aprevious transmission data block.

The following description is made with the appreciation that an HARQprocess unit is referred to as an “HARQ process block”, or simply an“HARQ process” unless it causes obscurity. An Identifier (ID) thatidentifies an HARQ process is referred to as an HARQ process number.

Now a detailed description will be made of a method for applying MIMO touplink signal transmission of a UE according to an exemplary embodimentof the present invention.

In accordance with the exemplary embodiment of the present invention, itis assumed that the UE acquires precoding information from a schedulinggrant signal received from a Node B, for use in uplink signaltransmission. In other words, the Node B transmits PMI information andMIMO signal transmission information by a scheduling grant signal to theUE, for use in uplink signal transmission.

In an asynchronous HARQ system characterized by a variableretransmission timing, the UE needs to receive a scheduling grant signalfrom the Node B, for every uplink signal transmission. In this case, theUE may acquire precoding matrix information from the scheduling grantsignal and precodes an uplink signal based on the precoding matrixinformation, prior to transmission to the Node B in accordance with theexemplary embodiment of the present invention.

Meanwhile, a synchronous HARQ system that presets a retransmissiontiming does not require the UE to receive a scheduling grant signal fromthe Node B, for each uplink retransmission. Accordingly, there exists aneed for a method for efficiently acquiring precoding matrix informationthat will apply to a particular retransmission HARQ process block,taking into account the feature of the synchronous HARQ system in theUE, which will be described below in detail.

Further, when the system employs semi-persistent scheduling (SPS), suchas the system providing services for VoIP (Voice over InternetProtocol), the BS sets transmission parameters such as RB allocation andMCS semi-statically. In SPS operation, a BS does not transmit schedulinggrant signal for initial uplink transmission and may transmit schedulinggrant for a retransmission if necessary. Accordingly, there exists aneed for a method for efficiently acquiring precoding matrix informationthat will apply to a particular retransmission HARQ process block,taking into account the feature of the SPS system in the UE, which alsowill be described below in detail.

FIG. 3 illustrates methods for transmitting an uplink signal in a MIMOscheme in a UE according to exemplary embodiments of the presentinvention.

FIG. 3( a) illustrates a case of receiving a scheduling grant signalfrom a Node B, for every uplink transmission in a UE, as in theasynchronous HARQ system, and FIG. 3( b) illustrates a case of notreceiving a scheduling grant signal from a Node B, for an uplinkretransmission in a UE, as in the synchronous HARQ system.

Referring to FIG. 3( a), the UE may receive a scheduling grant signalfrom the Node B, for an uplink signal transmission, in step S301 a. Thereceived scheduling grant signal may include precoding matrixinformation (e.g. PMI₁) for an upcoming uplink signal transmission ofthe UE. In step S302 a, the UE may transmit an uplink signal to the NodeB using an acquired precoding matrix (e.g. PMI₁).

If the Node B fails to receive the uplink signal in step S302 a, it maytransmit a Negative ACKnowledgment (NACK) to the UE in step S303 a. Atthe same time, the Node B may transmit a scheduling grant signalincluding precoding matrix information (e.g. PMI₂) to the UE, for aretransmission of the HARQ process in step S303 a. Upon receipt of thescheduling grant signal, the UE may retransmit the HARQ process using aprecoding matrix (e.g. PMI₂) indicated by the scheduling grant signal instep S304 a.

The system operating in the manner described in FIG. 3( a) may be anasynchronous HARQ system. Notably, when a synchronous HARQ system isconfigured so as to transmit a scheduling grant signal from the Node Bto the UE for every uplink retransmission, it may operate in the mannerdescribed in FIG. 3( a) in an exemplary embodiment of the presentinvention.

However, when the Node B does not transmit a scheduling grant signal tothe UE for every uplink retransmission in the synchronous HARQ system,the following problem may occur.

Referring to FIG. 3( b), the UE receives a scheduling grant signal foran initial uplink signal transmission from the Node B in step S301 b.The scheduling grant signal includes precoding matrix information (e.g.PMI₁) for use in the initial uplink signal transmission. As in theasynchronous HARQ system, the UE always receives a scheduling grantsignal from the Node B, for an initial uplink transmission in thesynchronous HARQ system. Subsequently, the UE may transmit an uplinksignal using a precoding matrix (e.g. PMI₁) indicated by the schedulinggrant signal in step S302 b.

If the Node B fails to receive the uplink signal in step S302 b, it maytransmit a NACK to the UE in step S303 b. The Node B may not transmit ascheduling grant signal to the UE, for retransmission of the HARQprocess. Then the UE needs a method for determining precoding matrixinformation for use in one or more subsequent retransmissions in stepS304 b.

According to another embodiment of the present invention for the systememploying SPS, the Node B does not transmit scheduling grant signal tothe UE for initial transmission and may transmit scheduling grant signalto the UE for retransmission signal if necessary. Thus, the UE needs amethod for determining precoding matrix information for use in one ormore retransmissions, similar to FIG. 3( b).

FIG. 4 is a flowchart illustrating a method for acquiring precodingmatrix information in a UE, when the UE fails to receive a schedulinggrant signal from a Node B according to an exemplary embodiment of thepresent invention.

Referring to FIG. 4, the UE determines whether a scheduling grant signalhas been received and a retransmission PMI has been acquired from thescheduling grant signal, for an uplink retransmission in step S401. Ifthe Node B transmits a PMI (e.g. PMI₁) by a scheduling grant signal forevery uplink retransmission even in the synchronous HARQ system as inthe foregoing exemplary embodiment of the present invention, the UE mayretransmit data using PMI₁ successfully.

However, if the UE does not receive PMI information for theretransmission from the Node B, it determines whether there is a latestscheduling grant signal including a PMI (e.g. PMI₂) for an HARQ processwith the same number as a current HARQ process to be retransmitted instep S402. In general, a retransmission using a latest received PMI forthe same HARQ process may be carried out successfully in MIMO withoutmuch performance degradation.

However, it may occur that there is not a latest received PMI for thesame HARQ process. That is, the UE may initiate a new transmission in adifferent HARQ process during the time period between the initialtransmission and the retransmission. Then the UE needs to consider thefollowings to select a PMI for the retransmission. The PMI selection mayvary depending on an algorithm that the Node B uses to calculate a PMIin transmission of a scheduling grant signal.

First of all, it is assumed that the Node B calculates the same PMI fora total system band (Wide band PMI). In this case, the PMI may not bechanged according to the size or position of resources allocated by aNode B scheduler. Thus, the retransmission may be carried out withoutperformance degradation by use of a PMI set in a latest scheduling grantsignal indicating a rank identical to the rank of a currentretransmission signal.

Specifically, the UE may determine whether there is a PMI indicated by alatest scheduling grant signal received for a signal of the same rank asa current HARQ process to be retransmitted in step S403. If the PMIexists and it equally applies to the total system band, the UE mayperform the retransmission using the PMI (e.g. PMI₃).

Meanwhile, the Node B may calculate the same PMI only for a frequencyband to be used for the receiver, not for the system band. The PMI isnot viable if the size or position of resources indicated by a currentscheduling grant signal is changed from that indicated by a previousscheduling grant signal. Even though the size or position of resourcesis not changed, the same PMI is not available when a rank is different.Thus the UE may use a PMI indicated by a latest scheduling grant signalthat allocates the same size or position of resources and/or the samerank.

Therefore, if a latest received PMI exists for the same-rank signal butthe PMI does not apply uniformly to the total system band in step S403,the UE additionally determines whether the PMI is for a signal identicalto the uplink signal to be retransmitted in the size and position ofresources (e.g. frequency band) in step S404. If the PMI is for a signalhaving resources of the same size and position, the UE may perform theretransmission using the PMI (e.g. PMI₃). On the other hand, in theabsence of the PMI, the UE may perform the retransmission using a PMI(e.g. PMI₄) indicated by a scheduling grant signal received for aninitial uplink transmission in the exemplary embodiment of the presentinvention.

For the most part, a MIMO system performs best when a channel does notchange rapidly. If the UE does not receive a scheduling grant signal fora retransmission, it uses the same frequency resources as used for aninitial uplink transmission in the exemplary embodiment of the presentinvention. Therefore, unless a channel changes fast, the use of the PMIused in the initial transmission for a retransmission does not affectperformance much.

Meanwhile, if there is no latest received PMI for an HARQ process of thesame rank in step S403, the UE uses the PMI (e.g. PMI₄) indicated by thescheduling grant signal received for the initial uplink transmission, asillustrated in FIG. 4. Notably, if a precoding codebook satisfies thenested property, the UE may also use a precoding matrix of the sameindex under a higher or lower rank. The nested property means that aprecoding matrix of a specific rank includes a precoding matrix of alower rank. When the codebook satisfies the nested property, the UE mayuse a PMI indicated by a latest scheduling grant signal despite adifferent rank.

It may be further contemplated as another exemplary embodiment of thepresent invention that when a UE does not receive a scheduling grantsignal for a retransmission, it uses a PMI preset between thetransmitter and the receiver. The PMI may be preset in many ways.

For example, a predetermined precoding matrix may be selected from a setof precoding matrices of an intended rank. One or more precodingmatrices may be selected. In case of selecting a plurality of precodingmatrices, the UE uses the PMIs indicating the precoding matricescyclically in a predetermined order. For instance, the order may bedefined as a function of fixed timing such as system timing.

On the other hand, another embodiment of the present invention,regarding the system using SPS, permits the UE to use the PMIinformation received via RRC (Radio Resource Configuration) signaling.Node B in the system using SPS does not transmit scheduling grant signalto the UE even for the initial transmission. In this case, the UE cannotacquire the latest PMI information or the like. Thus, the UE accordingto the present embodiment use PMI information received via RRCsignaling, and transmit uplink signal (e.g. VoIP packet) using thisinformation. Additionally, in SPS operation, if UE receives a schedulinggrant in a retransmission, UE retransmits using the parameters in thescheduling grant. After receiving a scheduling grant, the method todetermine PMI for the subsequent retransmission is the same as thepreviously explained method.

As is apparent from the above description, when a UE transmits an uplinksignal in MIMO, the UE can efficiently acquire precoding information andtransmit the uplink signal based on the precoding information.

The foregoing exemplary embodiments of the present invention areapplicable to a variety of future-generation wireless communicationsystems using MIMO for uplink signal transmission.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions.

What is claimed is:
 1. A method for performing a retransmission at aUser Equipment (UE) in a wireless communication system, the methodcomprising: retransmitting at least one data block to a Base Station(BS), wherein, if an uplink scheduling grant for the at least one datablock is not detected, then the at least one data block is retransmittedusing a precoding matrix determined based on whether a number of the atleast one data block is equal to a number of data blocks indicated bythe most recent uplink scheduling grant or not.
 2. The method accordingto claim 1, wherein, if the number of the at least one data block isequal to the number of data blocks indicated by the most recent uplinkscheduling grant, the precoding matrix is indicated by precodinginformation included in the most recent uplink scheduling grant.
 3. Themethod according to claim 1, wherein, if the number of the at least onedata block is not equal to the number of data blocks indicated by themost recent uplink scheduling grant, the precoding matrix is one ofprecoding matrixes for a number of layers corresponding to the at leastone data block.
 4. The method according to claim 1, wherein the wirelesscommunication system comprises a synchronous Hybrid Automatic RepeatReQuest (HARQ) system.
 5. A method for performing a retransmission at aBase Station (BS) in a wireless communication system, the methodcomprising: receiving at least one data block retransmitted from a UserEquipment (UE), wherein, if an uplink scheduling grant for the at leastone data block is transmitted, then the received at least one data blockis precoded using a precoding matrix determined based on whether anumber of the at least one data block is equal to a number of datablocks indicated by the most recent uplink scheduling grant or not. 6.The method according to claim 5, wherein, if the number of the at leastone data block is equal to the number of data blocks indicated by themost recent uplink scheduling grant, the precoding matrix is indicatedby precoding information included in the most recent uplink schedulinggrant.
 7. The method according to claim 5, wherein, if the number of theat least one data block is not equal to the number of data blocksindicated by the most recent uplink scheduling grant, the precodingmatrix is one of precoding matrixes for a number of layers correspondingto the at least one data block.
 8. The method according to claim 5,wherein the wireless communication system comprises a synchronous HybridAutomatic Repeat request (HARQ) system.